The lift-generating rotor blades of a rotor for a rotorcraft, in particular a helicopter, are deflected in various directions during continuous rotor operation, in particular by flapwise and lead-lag motions, and heavily stressed as a result. Rotor blades nowadays are manufactured predominantly from fiber composite materials.
In a bearingless rotor according to the existing art, as shown in FIG. 14, bearingless rotor blades are mounted on the rotor head usually via a rotor-head plate. The rotor-head plate has rotor-head-side blade connectors, embodied in accordance with the number of rotor blades, that are each joined to a structural element of a rotor blade. This structural element 142 is embodied, at a radially inner (with reference to the rotor disc) end of the rotor blade, i.e. the end facing toward the rotor head, with a rotor-head-side blade connector 144 that makes possible a connection to the rotor head. The transition from this blade connector 144 to the lift-generating rotor-blade regions is embodied as a blade neck 146. Structural element 142 transfers the drive torque from a rotor mast and the rotor head to the rotor blade. Structural element 142 furthermore transfers the centrifugal forces of the rotor blade to the rotor head. To allow structural element 142 to be separately fabricated and replaced in the event of damage, a separate disconnect point is often incorporated between structural element 142 and the rotor blade. The lift-generating rotor-blade region extends from this disconnect point to the outermost end, i.e. the blade tip, of the rotor blade. Serving as the disconnect point are, for example, at least two respective bolts that engage on the blade-end and rotor-head-side blade connectors. In FIG. 14, the rotor blade in the rotor-blade-side blade connector 144 is connected to the rotor head via two bolts 148. The centrifugal forces and the lead-lag moment are discharged via bolts 148. The flapping moment is also discharged via these bolts 148, usually assisted by an upper and a lower contact surface of structural element 142 on the rotor-head plate.
Blade neck 146 of structural element 142 of a bearingless rotor blade, which in the present technical field is also referred to as a flex beam and is enclosed by a so-called control bag 150, usually possesses a lead-lag-soft region that permits motions of the rotor blade in the lead-lag direction. The lead-lag-soft region thus constitutes a fictitious vertically oriented axis (also called a virtual lead-lag hinge) about which the rotor blade executes forward and backward lead-lag motions. In addition, blade neck 146 of structural element 142 usually has a flapwise-soft region that enables flapping of the blade in the vertical direction. The flapwise-soft region thus constitutes a fictitious horizontally oriented axis (also called a virtual flapping hinge) about which the rotor blade executes upward and downward flapwise motions. The distance between the virtual flapping hinge and the rotor axis of the rotor mast is referred to as the flapping hinge distance.
In a bearingless rotor, this flapping hinge distance is relatively large. The flapping hinge distance is, for example, approximately 8 to 12% of the rotor-disc radius, measured from the rotor axis of the rotor mast radially outward to the blade tip. A large flapping hinge distance in a bearingless rotor results, during operation, on the one hand in good helicopter control response and maneuverability, but on the other hand, in particular, in a high natural flapping frequency. This relative high natural flapping frequency, and the vibrations that result therefrom in the bearingless rotor, are disadvantageous in terms of the helicopters flying characteristics, and lead to large stresses on blade connector 144 and blade neck 146. Blade connector 144 and blade neck 146 must therefore have correspondingly large dimensions in order to withstand the stress that occurs. In conventional helicopter rotors, a low natural flapping and lead-lag frequency is desirable for these reasons.
Because of the large stresses on the rotor blade in a bearingless rotor, and the strength of those components that must therefore be ensured, it is extremely difficult to reduce the flapping hinge distance or decrease it below a specific value. In conventional bearingless rotors, a small flapping hinge distance would considerably reduce the durability and service life of the rotor blade in question, which of course is disadvantageous or even hazardous. On the other hand, however, a small flapping hinge distance would be desirable for a variety of applications, since helicopters having such rotor blades are generally perceived by pilots, crew members, and passengers as being more comfortable.
In special rotors, for example tilting rotors (so-called tiltrotors) of tiltrotor helicopters or aircraft, a particularly lead-lag-stiff rotor is required for various reasons.